1. Field of the Invention
The present invention relates to boating, and more specifically to a trim tab shape control system for trim tabs located at the stem of the boat to control the flight attitude of the boat.
2. Description of the Related Art
Trim tabs have been used for a long time to change the “attitude” of a boat. Attitude is the angle of the boat relative to the water surface, and changes under different operating conditions. The attitude of a boat relative to the water surface has a profound effect on the speed and efficiency of the boat. Attitude is usually discussed in terms of the nose up/nose down adjustment of the boat, and is sometimes called the trim angle. The term “trim angle” often leads to ambiguity as to whether it is the angle of the boat, or the trim tabs, or the outdrive being discussed. The present disclosure attempts to be more specific in discussing trim angle.
Trim tabs are usually fastened to the boat at or near the stem and on the transom or on the bottom of the hull. The transom generally forms the rearmost portion of the stem, such as the generally flat and vertical rearward end of the hull of the boat. When underway, water rushes under the boat, causing the rear of the boat to be deflected up or down by the trim tabs. Pushing the trim tab down deflects the departing water downward to boost the rear of the boat up into the air slightly, thus bringing the bow of the boat down. Pulling the trim tab up is intended to pull the rear of the boat down and to bring the bow up. Thus, “down on the trim tabs” means “down on the bow” and, conversely, “up on the trim tabs” means “up on the bow”. It is noted, however, that the “up on the trim tabs” operation of prior art trim tabs has limited effectiveness. Attitude may sometimes be discussed in terms of the left or right lean of a boat under way. Leaning may be due to propeller torque, uneven weight distribution, or cornering. Trim tabs may also be used to correct this leaning.
Although trim tabs are an appurtenance to the hull, they serve to modify the shape of the planing surface and, therefore, from the perspective of hydrodynamics of the boat planing on the water, it is immaterial whether the trim tabs are considered as part of the hull or an appurtenance.
Prior art trim tabs are only somewhat effective in changing the attitude of the boat. Early prior art trim tabs were hinged where they joined the hull of the boat and usually were a rigid flat plate essentially parallel to the bottom surface of the hull. This flat plate could swing up or down several degrees via mechanical means. A major deficiency of flat plate hinged trim tabs is that the up tab position causes an abrupt change in the contour of the surface running on the water. This abrupt change causes flow separation at the hinge point. As with any airfoil, flow separation causes loss of lift. The hinged flat plate is simply a crude airfoil with poor lift to drag ratio and is not very successful at raising the nose of the boat. Hinge type trim tabs in the down position will lower the bow of the boat, but have a poor lift to drag ratio and tend to impose excessive drag in order to generate an equivalent amount of lift of present invention.
More recent prior art trim tabs are of a bending flat plate type whereby the trim tab is a resilient plate of uniform thickness and stiffness. A flat plate is attached solidly in a cantilever fashion to the boat hull and does not use a hinged joint but rather relies on the bending of the flat plate slightly up or down to generate a somewhat better, but still deficient, approximation of an airfoil. The bending flat plate trim tabs were flexed down and up by the boat operator to add hook or rocker as desired. Hook is usually caused by a concave surface on the bottom of the boat, when viewed from below the boat, which tends to lower the bow while underway. Rocker is usually caused by a convex surface on the bottom of the boat, when viewed from below the boat, which tends to raise the bow of the boat while underway. The bending flat plate trim tabs were slightly more effective than hinged type plates. Although somewhat superior to hinged plate designs, the bending flat plate also has excessive drag for the amount of lift generated. Bending flat plate trim tabs are somewhat better than the hinged type in that the problematic abrupt change of angle of the hinge type is softened. This curved surface method decreases the tendency of flow separation, but uses a plate of constant flexural stiffness, so that the curvature is fairly localized at the point of attachment to the boat and diminishes as the water moves rearward away from the area of attachment of the plate to the hull.
Prior art trim tabs have now advanced to the use of a regressive flexural stiffness tab extending out rearward from the transom in a cantilevered fashion. This minimizes boundary layer separation in the up position by assuming the shape of a curve similar to a parabola. This shape improves the effective lift to drag ratio of the trim tab over earlier prior art. This recent prior art trim tab has an equal stiffness in the up direction as in the down direction. Excessive downward motion will cause the trim tab to break off due to fatigue failure or ductile failure.
It is desirable to have greater downward deflection capabilities than upward deflection capabilities. A trim tab shape control system according to an embodiment of the present invention is an improvement over U.S. Pat. No. 6,823,812 issued Nov. 30, 2004 to James P. von Wolske (hereinafter noted as “von Wolske 812”).